This invention relates to a fuel delivery system associated with an aircraft gas turbine engine and, more particularly, to a fuel delivery system which includes heat exchanger means for maintaining the temperature of the fuel and engine oil below limits which if exceeded would reduce the life and reliability of fuel and oil system components.
The invention herein described was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85 - 568 (72 Stat. 435; 42 USC 2457).
It is well known in the art to associate heat exchangers with gas turbine engine fuel delivery systems to provide for heat transfer from a hot fluid, such as oil from the engine lubrication system, to the relatively cool pressurized fuel delivered by the fuel delivery system to the engine fuel nozzles. The cooled engine oil is then better adapted to lubricate various selected components of the gas turbine engine. However, in certain fuel delivery systems under certain conditions, the effectiveness of such heat exchangers may have a deleterious effect upon the other engine components. Typically these adverse effects are encountered at specific points in the operating cycle of the gas turbine engine when the fuel, passing through the fuel pump associated with the fuel delivery system, exhibits a high temperature rise. When additional heat is transferred from the oil to the fuel in the heat exchanger, the high temperature of the fuel delivered to the engine fuel nozzles may result in gumming and coking of critical clearances and orifices associated with proper flow distribution in the engine combustors. Since the oil cannot be cooled to a temperature lower than the temperature of the fuel exiting the pump, the oil leaves the heat exchanger at a high temperature. The hot oil is then poorly adapted for lubricating the aforementioned selected engine components. The fuel delivery system described in a copending U.S. patent application Ser. No. 634,452 filed Nov. 24, 1975 is one fuel delivery system in which high fuel temperature rise may be encountered under certain engine operating conditions. The fuel delivery system described in this referenced application is comprised of a boost pump element in serial flow relationship with a primary centrifugal pump. Fuel is initially pressurized by the boost pump element, then additionally pressurized by the primary pump for delivery to the engine fuel control. The primary pump is drivingly connected to a power source, such as a gearbox drive assembly, while the boost pump element is selectively driven by a fluid motor under engine operating conditions and by the same power source as the primary pump under engine start-up and shutdown conditions.
The boost pump element and the primary pump are primarily designed for high flow and high pressure rise requirements of the engine at take-off power settings. At these high power settings, the temperature rise of the fuel through the delivery system is not significant since the thermal absorption capacity due to the high flow rate of the fuel is sufficient to absorb heat generated by inefficiencies of the system. Hence at high power settings, the fuel emerging from the delivery system may be appropriately passed through a heat exchanger and used to cool another fluid such as oil from the engine lubrication system. However, under engine start-up and shutdown conditions, which may exist for prolonged periods of time, the fuel demand of the engine fuel control is substantially less than the fuel demand at a take-off power setting. At the same time, the boost pump element is driven in an over-speed condition due to the direct drive connection with the primary pump power source. The boost pump over-speed condition and the low fuel flow rate through the boost pump under engine start-up and shutdown lead to an excessive fuel temperature rise across the booster pump element.
The temperature rise of the fuel across the fuel delivery system is also increased by the poor efficiency of the primary pump at low flow rates. The poor efficiency and associated temperature rise occur because the primary pump, which is designed and sized for high flow rates, is grossly oversized at low flow rates. The primary pump diffuser and blade configuration also tend to be mismatched at low flow rates. Consequently the primary pump generates significant heat which causes a temperature rise in the fuel which is excessive. The fuel is then subsequently passed through a heat exchanger encountering a further rise in temperature as the result of the transfer of heat from the oil. The hot fuel nozzles may be affected as previously described by the excessive temperature of the fuel and the hot engine oil may cause premature failure of the lubricated components of the engine.
Therefore it is an object of the present invention to provide an improved fuel delivery system wherein the temperature of the fuel delivered to the engine fuel nozzles is maintained sufficiently low so as not to deleteriously affect the flow distribution of the combustor nozzles.
It is a further object of the present invention to provide a fuel delivery system including heat exchanger means wherein the transfer of heat between the fuel and oil in the heat exchanger will prevent oil being delivered to the engine lubrication at an excessive temperature.
These and other objects, which will become apparent from the following specification and appended drawings, are accomplished by the present invention, which briefly stated in one form, provides a fuel delivery system associated with a gas turbine engine wherein the system includes a fuel supply reservoir supplying fuel to pump means which pressurize the fuel for delivery to engine fuel nozzles through a first fluid passage providing fluid communication between a first fluid outlet in the pump means and the nozzles. A second fluid passage is included for providing for return flow of the fuel from a second fluid outlet in the pump means to the supply reservoir. First heat exchanger means are operatively associated with the first fluid passage for providing for the transfer of heat between fuel flowing in the first passage and a second fluid. Second heat exchanger means are operatively associated with the second fluid passage for providing for the transfer of heat between fuel flowing in the second passage and the second fluid. Valve means are operative in a first mode to provide for flow of the second fluid through the second heat exchanger and further operative in a second mode for bypassing the second fluid around the second heat exchanger. The valve means are operative in the first mode when the pump means delivers fuel to the engine fuel nozzles at a low rate of fuel flow. The valve means are operative in the second mode when the fuel is delivered at a high rate of fuel flow.